diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-assets-fixture/assets/default.services.yml b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-assets-fixture/assets/default.services.yml
index cdfdd0a9f6..cc773f3e79 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-assets-fixture/assets/default.services.yml
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-assets-fixture/assets/default.services.yml
@@ -1 +1,4 @@
 # Test version of default.services.yml from drupal/core.
+# Add a dummy key until YamlPecl can validate an empty YAML file:
+# https://www.drupal.org/project/drupal/issues/3003300
+foo: bar
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-profile/assets/profile.default.services.yml b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-profile/assets/profile.default.services.yml
index e59669248f..2a35c02dba 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-profile/assets/profile.default.services.yml
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-profile/assets/profile.default.services.yml
@@ -1 +1,4 @@
 # default.services.yml fixture scaffolded from "file-mappings" in drupal-project composer.json fixture.
+# Add a dummy key until YamlPecl can validate an empty YAML file:
+# https://www.drupal.org/project/drupal/issues/3003300
+foo: bar